Small cell lung cancer is of neuroendocrine cell phenotype. A characteristic of small cell lung cancer is the establishment of an autocrine loop that stimulates cell proliferation. Small cell lung cancer expresses bombesin-like neuropeptides and the cell surface receptor for the neuropeptide, establishing a mechanism where the cell continuously stimulates itself to proliferate. The receptors for the bombesin-like peptides are thought to be coupled to heterotrimeric G proteins of the Gi and Gq family. Signal transduction via the Gi and Gq proteins initiate a number of early signalling events that drive proliferation. In this proposal the signal transduction pathways persistently activated in small cell lung cancer will be defined. the requirement for Gi and/or Gq will be determined using gene transfer techniques to introduce dominant negative G alphai and alphaq mutant subunits. The dominant negative alphai and alphaq mutant subunits will selectively inhibit receptor-coupled pathways regulated by the two different G proteins, allowing dissection of the Gi and Gq-regulated signal transduction pathways involved in small cell lung cancer proliferation. A similar genetic strategy will be used to define the cytoplasmic serine/threonine protein kinases whose activity is regulated by the mitogenic signals initiated from the activation of bombesin-like peptide receptors. Among the kinases to be analyzed are the growth factor-regulated mitogen activated protein kinase (MAP kinase), Raf and RSK90. Dominant negative mutants of the protein kinases that are constitutively activated will be expressed in small cell lung cancer to determine if they inhibit transformation and cell growth. Cumulatively, the experiments will define the signal transduction pathways that drive the transformed phenotype of small cell lung cancer. In neuroendocrine tumors of non-lung origin, activating mutations in G protein alpha subunits that drive cell proliferation have been identified. Small cell lung cancer lines and tumors from lung cancer patients will be screened for activating mutations in alphai and alphaq. In addition, SCLC tumors will be screened for activating mutations in the bombesin-like peptide receptor genes. It is predicted that activating mutations in the receptors of G proteins would contribute to the progression of neoplastic transformation in small cell lung cancer. Defining the activating mutations and signal transduction mechanisms causing constitutive cell proliferation will ultimately define a strategy for direct pharmacological intervention in small cell lung cancer. Using receptor antagonists and inhibitors of the effector systems activated by the receptor-G protein- coupled pathways should allow complimentary intervention of mitogenic signals for successful growth arrest of small cell lung cancer.